1. Field of the Invention
The present invention relates to, for example, combustion chambers suitable for relatively small two cycle internal combustion engines which are used for portable machines and the like. In particular, it relates to combustion chambers for two-cycle internal combustion engines having reduced noxious substances in the exhaust gas without impairing output characteristics of the engine.
2. Description of Prior Arts
Exhaust gas is regarded as a principal cause of air pollution which is present in exhaust gas from boilers used in factories and internal combustion engines in automobiles. Various attempts have been made for reducing noxious substances contained in such exhaust gas, i.e., NOx (substances resulting from combustion of fuel), CO (substance resulting from incomplete combustion of fuel), HC (substance of unburnt combustible from fuel), and the like.
However, little attention has been directed toward relatively small (total displacement is about 35 cc or less) general purpose two-cycle gasoline engines which are used for portable machines, and the like, because of their small displacement. Recently, however, in view of increased concern with environmental problems, increased attention is being focused on small two-cycle internal combustion engines with a view to reduce noxious substances in exhaust gas discharged therefrom.
FIGS. 3 and 4 show one illustrative form of a cylinder of a conventional small air-cooled two-cycle gasoline engine, for which it is urgently needed to reduce noxious substances in exhaust gas.
Referring to FIGS. 3 and 4, the cylinder 50 comprises a cylinder portion 2 having a cylindrical inner wall 5 into which a piston is fit-inserted and a head portion 3 formed with a combustion chamber 7 of so-called squish dome type having a ring-shaped squish surface 7d integrally formed with each other, and having a plurality of cooling fins 12 at its peripheral portion. In the cylinder portion 2, an intake port 14 and an exhaust port 16 are so formed as to open oppositely at different levels, and a pair of scavenging ports 20, 20 formed on opposite sides and which employ a loop scavenging mode symmetrical with respect to the longitudinal section bisecting the exhaust port 16 (the vertical plane passing along the axis CL of the cylinder 1) [Schneule scavenging mode].
In this form, the combustion chamber 7 is formed with an ignition plug mounting opening 8 on the side opposite to the exhaust port 16 as shown in FIG.3, and a top wall surface 7b, which is slightly concave upwardly and is positioned adjacent to the top inner edge of the ignition plug mounting opening 8, and progresses to the end of the top wall surface 7b opposite to the exhaust port 16. A middle wall surface 7c is formed opposite to the ignition plug mounting opening 8, which sharply descends defining a circular arc-like curved surface in its cross-section, and a squish surface 7d extending downwardly is formed into an inclined surface having a relatively gentle gradient relative to the top end 5a of the cylinder portion 2 of the cylinder 1 and skirts the lower edge of the combustion chamber 7. Consequently, a scavenging air stream which has been ascending along an inner wall 5 opposite to the exhaust port 16 in a conventional cylinder 50 cannot effect sufficient scavenging with respect to the inner portion of the combustion chamber 7, because the stream is deflected at the squish surface 7d of an intake 7a to the combustion chamber 7. Further, the peripheral area of the top end 5a of the cylinder portion 2 is likely to be outside the scavenging air stream.
In conventional small two-cycle engine using gasoline as fuel, the presence of noxious substances contained in exhaust gas discharged therefrom such as NOx and CO can be reduced to some extent by optimizing air/fuel ratio or the like. However, no effective measures for reducing HC have been found because of insufficient scavenging performance.